Purpose. This study assessed the prevalence of 25 clinically important drug-drug interactions (DDIs) in the ambulatory care clinics of the Department of Veterans Affairs medical centers (VAMCs). Methods. This study was a retrospective, cross-sectional database analysis of pharmacy records to assess the prevalence of 25 clinically important DDIs. For each DDI, the object drug was defined as the medication that has its therapeutic effect modified by the drug interaction process. The precipitant drug was defined as the medication responsible for affecting the pharmacologic action or the pharmacokinetic properties of the object drug. Rates of interactions for each VAMC facility were calculated as the number of patients with a DDI divided by the total number of individual patients exposed to the object or precipitant medications. The 25 DDIs were categorized into four main categories on the basis of the therapeutic classification of the medications involved in the drug pairs. Results. The study population included 2,795,345 patients who filled prescriptions for medications involved in potential DDIs across 128 VAMCs. The highest DDI exposure rate was 129.2 per 1,000 recipients of monoamine oxidase inhibitors (MAOIs) that occurred with combinations of selective serotonin-reuptake inhibitors (SSRIs). The lowest DDI exposure rate was 0.01 per 1,000 warfarin recipients who had the warfarin and sulfinpyrazone combination. Conclusion. The analysis of pharmacy records of veterans who filled prescriptions at the outpatient settings within VAMC found an overall rate of 2.15% for potential DDIs. Case-exposure rates were greatest for veterans receiving SSRIs and MAOIs, ganciclovir and zidovudine, anticoagulants and thyroid hormones, and warfarin and nonsteroidal antiinflammatory drugs.
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